More than one visitor to these relatively tiny, twisting waters have wondered how such a small stream could be responsible for the broad, deep, straight valley of Minnesota. As a matter of geological fact, the present
Minnesota River is not responsible. Another river, a swollen torrent of icy currents, filled the developing
Minnesota Valley and carved it a millennia ago. This was the Glacial River Warren, named for General G.K. Warren, Union officer in the Civil War and army engineer, who first described the origin of the wide valley. It flowed from the south end of Glacial Lake Agassiz some 9,000 to 12,000 years ago, south because the northern outlets to the Arctic and Atlantic oceans were blocked with ice. Today's Minnesota is termed by geologists as underfit river, a small stream flowing in the great valley cut by the Glacial River Warren.

The last glaciers in southern Minnesota area retreated northwest, and when the southeastern margin was near the area of present Twin Cities, the meltwaters flowed only a short distance to the lower Mississippi; its course was shaped by older moraines and the edge of the ice itself. As the ice continued to melt, the incipient Minnesota River worked its way upstream - first to the southwest and then (turning at present Mankato) to the northwest. Evidence of the river found far above the present valley floor, such as sandy terraces and potholes, attest to the presence of the river at a much higher elevation than that of today's stream. After the ice edge and the river headwaters reached the continental divide at Browns Valley and vast Lake Agassiz was formed to the north, River Warren began cutting the Minnesota Valley. Today, in some areas the valley is five miles wide, and it lies as much as 250 feet below surrounding plains, a reminder of the great volume of water that once raged down its course.

The high, steep bluffs along the sides have had a profound effect on the character of tributary streams. Rivers
and creeks winding slowly across the southern plains, or flowing down from the northern lake and hill regions through marshes and swamps, suddenly plunge into the Minnesota Valley. The results are varied but always interesting from both aesthetic and hydrological viewpoints. Indeed, some of the most delightful river environments in Minnesota are along the bluffs, rims and bottoms of the Minnesota River Valley. Plunging waters of the tributaries have formed rapids, waterfalls, and deep gorges.

Source:
Waters, Tom F; The Streams and Rivers of Minnesota